Sensors that observe inertia of an object such as acceleration or an angular velocity, a pressure variation caused by a substance such as sound waves and seismic waves, and a state of an object such as an inclination angle have been widely used for static and dynamic control of vehicles including automobiles, and acquisition of sound and earthquake information. In recent years, usage of the sensor systems using an MEMS technology have been expanding in combination with a reduction in size and a reduction in price of the sensor systems, such that a vibration sensor is leveraged for the purpose of acquiring maintenance information on aging infrastructure, various control are performed by leveraging information on an inertial sensor introduced to smartphones, and so on.
In the sensor systems as described above, detection principles of sensor elements have been known as a capacitance detection type, a piezo resistance type, and a heat detection type. In the capacitance detection type among those types, physical property values of silicon (Si) and so on are well known, and the MEMS structure applying a semiconductor microfabrication technology with the use of stable materials are employed with advantages that the capacitance detection type can be manufactured inexpensively and compactly with high affinity, and developed actively in universities and companies all over the world. For example, Patent Literature 1 discloses an example of a capacitance detection circuit that detects a change in electrostatic capacitance of a capacitance type sensor element.
The operation of the capacitance detection type sensor will be described with reference to FIG. 1. Now, a typical uniaxial MEMS acceleration sensor will be described as an example, but the capacitance detection type sensor is not limited to MEMS. Moreover, the detection principle of the capacitance detection type pressure sensor and the capacitance detection type angular velocity sensor is the same, and can be considered with the use of the same model.
As illustrated in FIG. 1, the capacitive acceleration sensor includes a frame FRAME, a movable mass MASS, springs kp and kn, dampers dp and dn, a positive side detection electrode ELP, a negative side detection electrode ELN, a positive side detection capacitor Cp, and a negative side detection capacitor Cn. The RRAME and the MASS are electrically connected to each other through the springs, but are electrically insulated from ELP and ELN. Moreover, the FRAME, the ELP, and the ELN are mechanically connected to each other, and those three components move simultaneously with a relative position fixed.
In an initial state, an initial capacity is Cp=Cn=C0. Naturally, Cp and Cn do not perfectly coincide with each other due to a processing accuracy, but in this example, an ideal state will be considered for description of the operation. When acceleration is applied, the FRAME moves in a direction indicated by an arrow in FIG. 1, and a relative position in the FRAME of the MASS changes in the direction of the arrow. Actually, the FRAME does not operate perfectly in one axial direction, but the idea state will be considered. The movement of the MASS is limited by the springs and the dampers and the MASS is not kept to move in one direction.
Since the ELP and the ELN are fixed to the FRAME, the Cp increases and the Cn decreases as a distance between the ELP and the MASS decreases. Conversely, as a distance between the ELP and the MASS increases, the Cp decreases and the Cn increases. As a change in the Cp and the Cn, a capacitance between a terminal memsinp connected to the ELP and a terminal memsout connected to the MASS and a capacitance between a terminal memsinn connected to the ELN and the terminal memsout are measured to detect acceleration. As a method of detecting the electrostatic capacitance, a method of changing an electric charge accumulated in a variable capacitor with the use of carrier waves and reading the electric charge is generally known. Referring to FIG. 1, the carrier waves of opposite phases are input from the respective terminals memsinp and memsinn, and the electric charge coming out from the terminal memsout is measured.
The present technology has been devised to achieve high sensitivity of a capacitance detection type sensor including the MEMS sensor as described above.